1. Field of the Invention
The invention is directed to an objective diaphragm for imaging an image original to be scanned line-by-line onto an optical CCD sensor.
2. Description of the Prior Art
Automatic optical character recognition systems for reading image originals (i.e., documents, forms and the like) can be fundamentally divided into two subsystems. The first subsystem is the acquiring and processing of the image original mensurationally, whereas the second subsystem is the actual image recognition. The acquisition system usually has an optoelectric scanner device (scanner) that can be divided into an optical part and a processing part.
The optical part has optical imaging components with which the image original to be scanned is imaged onto a line or surface sensor for image acquisition. For example, known line sensors include CCD sensors (Charge Coupled Devices) that can be used in various lengths and pixel sizes.
A quantity of light generated by an illumination system is supplied to the sensor via an objective in the imaging optics. This quantity of light, among other things, is determined by the aperture relationship of the objective that is defined by the aperture diameter and focal length thereof. To limit image errors in the optical imaging and to avoid stray light within the scanner device, the objective has a diaphragm. However, the diaphragm thus also limits the amount of light incident on the sensor.
Many conflicting demands are made of the imaging optics that cannot be simultaneously fulfilled. One example is the desire for an optimally large diaphragm aperture to achieve a high light intensity on the sensor and, thus, a high scanning speed. This goal is opposed by the requirement of having a small diaphragm aperture for achieving a high depth of field and thus, good scan quality.